1. Field of the Invention
The present invention relates to an optical pickup for recording and/or reproducing an information signal with respect to an optical disc, and also relates to an optical disc apparatus using the optical pickup.
2. Description of the Related Art
In optical pickups for recording and/or reproducing information signals with respect to optical discs, it has recently been proposed to use an objective lens as a component of the optical pickup, which is in the form of a plastic lens and a single lens, for the purposes of reducing the overall weight and the production cost.
Also, an optical disc has recently been used which enables information signals to be recorded at a higher density than in ordinary optical discs, e.g., a CD (Compact Disc) or a DVD (Digital Versatile Disc). For example, there is a BD (Blu-ray Disc (registered trademark)) in which signals are recorded and/or reproduced by using a light (optical) beam emitted from a violet semiconductor laser and having a wavelength of about 405 nm.
As in the optical pickup for the DVD and the CD, using a plastic lens as an objective lens is also desired in an optical pickup for use with such a high-density recording optical disc, e.g., the BD. However, because the plastic lens exhibits a larger variation in refractivity depending on temperature change than a glass lens, larger spherical aberration is generated in the plastic lens due to the temperature change. Particularly, when the lens has a high numerical aperture (NA) of about 0.85 as in the optical pickup for the BD, spherical aberration is generated to such an extent as greatly affecting the focusing performance of the lens even with a slight temperature change of about several degrees.
One of general methods for correcting the spherical aberration generated due to temperature change is to provide a diffraction structure on the surface of an objective lens, and generating spherical aberration in polarity opposed to that of the spherical aberration generated due to temperature change, thus canceling the latter spherical aberration. However, application of great diffraction power is necessary in order to completely correct the spherical aberration generated due to temperature change. This gives rise to a problem that the diffraction structure becomes so fine and a difficulty occurs in forming the diffraction structure. Another problem is that the necessity of machining a fine structure raises a difficulty in obtaining satisfactory light utilization efficiency due to incomplete cutting (generation of an uncut portion) of a mold.
Another conceivable method for correcting the spherical aberration generated due to temperature change is to drive a collimator lens in the direction of an optical axis so as to cancel the spherical aberration generated due to temperature change by spherical aberration of magnification (see Japanese Unexamined Patent Application Publication No. 2008-4169). This method is, however, contradictory to the intention to reduce the weight of optical pickup for the reason that the driving stroke of the collimator lens is very large and the size of an optical pickup is increased.
Further, the driving stroke of the collimator lens can be reduced by forming a divergent angle conversion lens as a two-element lens to increase sensitivity in generation of the spherical aberration that is generated with the lens driving in the direction of the optical axis, and to reduce the driving stroke of the collimator lens. However, this method increases the number of lenses and is contradictory to the intention to suppress the production cost.
The above-described methods additionally employ, in common, a system which monitors temperature and drives the lens depending on temperature change. Such a system, however, has a difficulty in executing accurate feedback system control because the sensitivity in generation of the spherical aberration is too high in view of executing the correction corresponding to the generated spherical aberration.
Further, the large spherical aberration generated due to temperature change includes spherical aberration components of fifth or higher orders, which are generated at rates different from higher-order components included in the spherical aberration of magnification generated with the lens driving. This results in a difficulty in completely canceling the spherical aberration components of fifth or higher orders.